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Navigation is key to national and international industry, commerce, and safety. Knowledge of position, both relative and absolute has been used throughout history to gain tactical advantage in both peaceful and not-so-peaceful pursuits. From the rudimentary techniques developed over two millennia ago, people all over the world have made both evolutionary and revolutionary progress in the business of knowing their position. Navigation progressed from simple piloting, the art of connecting known points, to satellite-based navigation systems.
Today the premier worldwide navigation solution is the Global Positioning System (GPS). This satellite-based navigation system was developed by the Department of Defense (DoD) to support a variety of military operations. This system has been utilized in a variety of civilian systems. As the adoption of satellite based navigation technology has grown since its introduction in the early 1980""s, the number and complexity of devices for personal navigation and location. GPS is broken down into three basic segments, as follows: 1) Space, comprising the satellites; 2) Control, incorporating tracking and command centers; and 3) User, performing navigation functions based on ranging to the satellites.
The space segment contains the GPS Space Vehicles (SV) placed in circular orbits with 55xc2x0 inclination, a semi-major axis of 26,560 km (20,182 km altitude) corresponding to an orbital period of 12 hours sidereal. There are six orbit planes placed at 60xc2x0 offsets in longitude with nominally four satellites in each plane, giving 24 satellites. Currently there are 28 active satellites in the planes. Spacing within the plane is adjusted to achieve optimal coverage over regions of interest. The satellites themselves are three-axis stabilized and use solar panels to provide power. Each satellite contains a pair of atomic clocks (for redundancy) which have a stability of 1 part in 1013. Each satellite broadcasts on two frequencies, 1575.42 MHz (L1) and 1278.6 MHz (L2). The L1 signal contains two separate pseudorandom noise (PRN) modulations: 1) the Clear Acquisition (C/A) code at bit or xe2x80x98chippingxe2x80x99 rate of 1.023 MHz (i.e., each millisecond there are 1023 modulated bits or xe2x80x98chipsxe2x80x99 transmitted); and 2) the so-called xe2x80x98Pxe2x80x99 code which has a chipping rate of 10.23 MHz or 10 times that of the C/A code. The L2 signal only contains the P code. GPS uses a PRN coding sequence of bits that have a specified length but have the property that different codes do not strongly correlate with one another (i.e., they are orthogonal). The C/A code is 1023 chips long and thus repeats every 1 millisecond. The full P code length is 38 weeks but is truncated to 1 week.
The control segment is responsible for the operation and maintenance of the Global Positioning System. There are five monitoring stations worldwide at Kwajalein, Hawaii, Colorado Springs, Diego Garcia and Ascension. These stations measure the discrepancies between the satellite state information (satellite positions and clock) as well as health of the satellites. The Master Control Station (MCS) in Colorado Springs formulates predicted values and uploads them to the satellites. This data is then included in the new message for broadcast to the users.
The user segment comprises GPS receivers that decode the satellite messages and determine the ranges to at least four GPS SVs to determine 3D position and the receiver clock offset. Users break down into two main groups: authorized and unauthorized. Authorized users have full access to both the C/A and P codes. Authorized users are restricted to the military and other special groups or projects with special permission from the DoD. Unauthorized users generally cannot access the P codes as the code itself is encrypted before broadcast by a process known as anti-spoofing (AS). This makes the process of emulating a GPS signal to the authorized user more difficult. The encrypted modulated signal is known as Y code. Additionally the hand-over-word (HOW) between the C/A and Y code is also encrypted. Authorized users are given a xe2x80x98keyxe2x80x99 that allows for the decryption of the HOW as well as the Y code. Authorized user receiver equipment with dual frequency code access utilizes what is known as the Precise Positioning Service (PPS).
GPS receivers are very sensitive devices capable of measuring the low signal levels available on, or near, the surface of the Earth. A GPS receiver design incorporates radio frequency (RF) elements, signal downconversion, signal sampling, digital signal processing, as well as computational devices and methods. Nominally, at least four timing measurements are combined to solve for a position solution and time offset from a given time reference at a given time (or epoch). Much GPS determination today determines a three-dimensional (3D) position of the receiver (see, for example, B. W. Parkinson et al., Global Positioning System: Theory and Applications, Volumes I and II, (Progress in Astronautics and Aeronautics, American Institute of Aeronautics and Astronautics, 1996)); that is, the latitude, longitude and altitude of the receiver are assumed to be unknown, and the GPS receiver determines its 3D position using what is referred to as a 3D-plus-clock solution.
Sometimes, the arrangement of the satellites being tracked provides poor geometry so an accurate 3D-plus-clock solution is impossible. In these cases, numerous alternatives are generally pursued today. As a first alternative, it is common practice to remove the altitude component from the position computation. The problem with the first alternative is that the user""s altitude information is not solved for, but is assumed by an a priori guess. This guess is either some fixed number (e.g., 0 meters altitude, sea level) or an xe2x80x9caltitude-holdxe2x80x9d using the last known altitude. This is an important drawback because altitude can change significantly over time and either fixed altitudes or altitude-hold modes can introduce errors not only in altitude but also in horizontal positioning.
In a second alternative (see, for example, U.S. Pat. No. 6,055,477), the position solution is augmented by separate altitude sensors. The second alternative for fixing the problem requires additional hardware, thus increasing the complexity and cost of the positioning device.
In a third alternative (see, for example, U.S. Pat. No. 6,061,018), the GPS receiver interfaces with a cellular communications system and the altitude of the GPS receiver is determined based on the altitude of the cell. However, this appears to limit the altitude used in the calculations to the single altitude associated with the cell (even though it may be an average of altitudes in the cell), regardless of the actual altitude of the GPS device.
There is a need to determine the position of a GPS receiver in a manner different from the traditional methods when the 3D-plus-clock solution is inaccurate.
The current invention overcomes the problem without the aforementioned drawbacks of the traditional methods. This invention uses an iterative approach to remove the true local altitude from the solution without adding additional hardware or cost to the remote location device.
In the present invention, the inaccurate 3D position can be used to localize the position well enough to get a crude altitude. This altitude forms the basis or xe2x80x9cguessxe2x80x9d to remove it from the 2D position estimation process. A new 2D position estimate will form the basis for the lookup into a table for altitude which is used for the next 2D position estimate, and so on. By utilizing altitude the horizontal position solution can be constrained to minimize the error in altitude.
According to one embodiment, a method according to the present invention includes the acts of receiving GPS measurements from a GPS device; calculating, as a three-dimensional solution, an initial position of the GPS device from said GPS measurements; determining, from a database, an initial altitude corresponding to an initial latitude and an initial longitude of the initial position; calculating, as a two-dimensional solution using the initial altitude, a revised position of the GPS device from the GPS measurements; and repeating the altitude determination and 2D solution calculation until the position converges.
These and other features of the invention are detailed in the following description and accompanying drawings.